1. Field of the Invention
The present invention relates to polymer compositions based on fluoropolymers, suitable especially for the manufacture of electrical cables, hollow bodies and flexible pipes intended, for example, for conveying fuels. It also relates to the cables, pipes, hollow bodies and other articles partially or wholly made from these compositions.
2. Description of the Related Art
When they are being used, electrical cables are commonly subjected to both mechanical and thermal stresses which are detrimental to the integrity of their insulation. This explains the usefulness of standards such as the qualification UL 444 (developed by the Underwriters Laboratories Inc.), which is an aging test consisting in subjecting the cables to an elevated temperature (136.degree. C. for the "125.degree. C. rating" and 158.degree. C. for the "150.degree. C. rating") for a number of days (7 to 150 days) and then verifying whether the elongation and the breaking stress have not changed by more than 50%. With a view to manufacturing electrical cables capable of satisfying these standards, one solution consists in sheathing the cables with a layer of PVDF (polyvinylidene fluoride), by itself or combined with other polymeric layers, for example based on polyethylene, PVC or other fluoropolymers. However, PVDF homopolymer is a resin of high rigidity and it is therefore preferable to employ copolymers of VF.sub.2 (vinylidene fluoride) and of other fluoromonomers such as HFP (hexafluoropropylene), which are more flexible. However, such copolymers exhibit the disadvantage of a melting temperature which is markedly lower than that of PVDF homopolymer, it being possible for the difference to reach 40.degree. C. It is clear, however, that the melting temperature of the compositions employed should be advantageously higher than the temperatures applied during the aging test described above.
Another application in which excellent thermomechanical properties are required is that of flexible pipes, and in particular that of the conduits for fuel circuits employed in motor-vehicles (fuel lines).
These conduits must conform to many requirements such as, for example:
good chemical resistance and imperviousness to the fuels employed, including those containing methanol, PA1 good impact strength, even at low temperatures, PA1 allowing use temperatures that can reach 100.degree. to 150.degree. C. PA1 (A) approximately from 25 to 75% of at least one PVDF homopolymer; PA1 (B) approximately from 25 to 75% of at least one thermoplastic copolymer of VF.sub.2 and of at least one other fluoromonomer, exhibiting a content of approximately 5 to 25% by weight of this other monomer. PA1 a) at least one PVDF homopolymer (A) in weight proportions of at least approximately 25%, preferably higher than 30% and not exceeding approximately 75%, preferably lower than 70%, particularly preferably lower than 45% and ideally lower than 40%; PA1 b) at least one thermoplastic copolymer (B) of VF.sub.2 (vinylidene fluoride) and of at least one other fluoromonomer, the weight content of this other comonomer in the copolymer being at least approximately 5%, preferably at least 10% and not exceeding approximately 25%, preferably 20%, particularly preferably 17%; this copolymer being present in the mixture in proportions of at least 25%, preferably higher than 30%, particularly preferably higher than 55%, ideally greater than 60% and not exceeding approximately 75%, preferably lower than 70% (by weight). By way of fluorocomonomers that can be employed there may be mentioned especially CTFE (chlorotrifluoroethylene), HFP (hexafluoropropylene) and TrFE (trifluoroethylene) and mixtures thereof. CTFE and HFP give very good results.
PVDF homopolymer excels insofar as permeability is concerned, but has a high rigidity and poor impact strength when cold (dart drop test at -40.degree. C.). Most polyamides (such as PA 12) have inverse properties: good resilience when cold but a permeability that is too high and, furthermore, a limited maximum temperature of use. Polyethylene also exhibits these two latter disadvantages.
A solution that could be envisaged would be to incorporate into PVDF one or more plasticizers intended to improve its mechanical properties at low temperatures, but the problem which then arises, in the case of the conduits for conveying fuels, is that of the extraction of these plasticizers by the fluids being conveyed, and this produces the risk of resulting in a gradual embrittlement of the conduits.
Alternatively, in Belgian Patent 832,851 (Dynamit Nobel A.G.), which relates to a PVDF--elastomeric fluorocopolymer (for example VF.sub.2 -HFP) mixture, in order to give it high resilience and elongation at break without excessively affecting its heat resistance (the Vicar point), it is strongly recommended to crosslink the elastomer and possibly then to postcure the articles thus produced. These operations naturally require the use of special additives (crosslinking agents, vulcanization accelerators) and of additional stages of manufacture. Furthermore, in this document there is a recommendation against exceeding a copolymer content of more than 30% by weight in such mixtures.
In the context of electrical cables as in that of flexible pipes the present invention is aimed at providing polymer compositions with improved thermomechanical properties which, furthermore, dispense with the use of a plasticizer. In addition, the compositions of the invention have the advantage of not requiring a subsequent crosslinking.